Cycloadditions have become a mainstay of organic synthesis in the preparation of a wide host of natural products and medicines. photochemical 2+2 cycloadditions provide an especially valuable synthetic tool because of the efficiency with which several bonds are formed and because of the variety of ring systems that can be constructed by further manipulation of the initial polycyclic photoproducts. This proposal seeks to explore the scope of a newly discovered photoinduced cycloaddition of alkenes to excited triplet benzenes. The initial photoproducts are bicyclo (4.2.0)octa-2,4- dienes resulting from ortho 2+2 addition. These are converted thermally to cyclooctatrienes, which undergo photoisomerizaticn to bicyclo(4.2.0) octa-2,7-dienes. Thus the overall chemistry can lead either to eight-membered rings or to two different bicyclic structures that would lead to a variety of alternative structures. The compounds that have been studied already are (ortho - and para- butenoxy)phenyl ketones. Primary specific research aims are: 1) define the mechanism of the intramolecular reaction; 2) explore a wide variety of tethers other than -O-(CH2)2--; 3) explore how other substituents on the benzene ring affect the regioselectivity of addition; 4) explore the reactivity of variously substituted phenyl ketones in bimolecular addition; 5) determine the regioselectivity of bimolecular addition to the variously substituted benzenes; 6) explore triplet sensitized reactions of compounds without acyl groups so as to broaden the reaction's usefulness; 7) determine the reactivity of heteroaromatics, which would lead to various heterocyclic products; 8) illustrate the reaction's usefulness by preparing several ring systems of current synthetic interest. The overall aims of the research are 1) to define how well this photochemistry may complement the cycloaddition of alkenes to cycloalkenones and to singlet excited benzenes as synthetic methodology; and 2) to develop a much better understanding of the reactivity of excited triplet benzenes.